Traditionally, the outermost well casing (commonly referred to as the conductor casing) in petroleum and gas wells is installed by a Mobile Offshore Drilling Unit (MODU) or drilling rig that will also complete drilling the well to final depth. The conductor casing, generally 30″ to 36″ diameter pipe, 200 ft to 600 ft in length, is the first well casing installed. There are a number of methods utilized for installing the conductor casing to final penetration depth including jetting, turbo-drilling, and hammering.
In the jetting process the conductor casing is lowered on the MODU's drill string. At the tip of the conductor casing a jetting fixture on the end of the drill string allows the vessel to pump water or other fluids down the drill string and through the jetting fixture in an action that washes away the soil underneath the tip of the conductor casing allowing it to penetrate the soil.
Turbo-drilling is a variation of jetting in that a so called mud motor is affixed to the end of the drill string at the tip of the conductor casing. When fluids are pumped down the drill string the mud motor rotates causing a large drill bit to rotate at the tip of the conductor casing. The drill bit removes soil allowing the conductor casing to penetrate the soil.
Hammering refers to use of a pile hammer deployed from the MODU to drive the conductor casing into the soil. Because there is much less disturbance of the soil by hammering the conductor casing it is less likely to experience subsidence problems and is considered by many in the industry to be the preferred method if cost, hammer handling and rigging issues are excluded.
Regardless of the method used to install the conductor casing by the MODU it generally accepted by the offshore oil industry that substantial cost savings can be realized by pre-installing the conductor casings prior to the arrival of the MODU. This allows the MODU to proceed at once with conventional drilling and casing activities once it arrives at the wellsite.
Conductor casing pre-installation has been preformed previously but only by the use of so called Construction Vessels. Examples of Construction Vessels include Semi-submersible Crane Vessels (SSCV), Multi-service Vessels (MSV), Diving Support Vessels (DSV), Derrick Barges and Pipe Lay Barges.
In accordance with the present invention a hydraulic pile driving hammer is deployed from the work deck of a non-construction vessel, specifically an Anchor Handling/Tug/Supply (AHTS) vessel. The procedures, devices and equipment needed to perform this action provide an economic advantage due to the fact that the AHTS vessel lease rates are traditionally much less than MODU and Construction Vessel lease rates. By way of example, typical day rates for the foregoing vessels are as follows:
SSCV:$250,000 to $500,000 per dayMSV:$150,000 per dayDSV:$100,000 to $250,000 per dayDerrick/Pipe Lay Barge:$250,000 to $500,000 per dayAHTS:$75,000 to $95,000 per day
A perceived advantage to both the AHTS and Construction Vessel approach is that the conductor casings are “batch set”, meaning many or all the conductor casings needed in a particular oil or gas field are installed in short duration of time. This allows the soil surrounding the conductor casing to reconsolidate or “setup”, thereby providing higher vertical load capacity and lessening the likelihood of subsidence.